Optimum nitrogen fertilizer rates to apply for corn can vary by more than 100% from field to field and year to year. This variability is a result of near limitless interactions among crop genetics, management and environment factors such as weather and soil type. Among these factors, the amount of inorganic nitrogen in the soil is one of the most important.
The amount of soil nitrate carried over from one year to the next can vary considerably, says Sotirios Archontoulis, an Iowa State University professor of integrated cropping systems.
With low amounts of soil nitrate entering the crop year, how does this affect nitrogen management application decisions farmers need to make this year? Archontoulis and his research associates Mitchell Baum and Amy Cooper provide the following information and observations to help answer that question:
Results of on-farm testing
Through the Iowa Nitrogen Initiative, a partnership between ISU and farmers hosting field research on their farms, the ISU team sampled 30 farmers’ fields in nitrogen trials spread across Iowa in 2024. They did this to quantify soil inorganic N levels in the fields.
“In each field, we measured soil inorganic N from soil depths of zero to 12 inches and 12 to 24 inches,” Archontoulis says. “We did this where the lowest [zero], middle and highest nitrogen fertilizer rates per acre had been applied for the 2024 crop.”
The soil nitrate sampling was conducted after harvest in the fall. Here’s what they found:
- In 96% of the fields, soil inorganic nitrogen levels in the top 12 inches of soil were less than 20 pounds of N per acre. For reference, this is the lowest level of residual soil nitrogen in the Nitrogen Fertilizer Application Consultation Tool (N-FACT) decision support system.
- In conventional soil tests such as the late spring soil nitrate test, 20 pounds of nitrate-nitrogen in the top 12 inches is about 6 parts per million.
- The 2024 post-harvest soil nitrate N levels ranged from 1.5 to 15.5 pounds per acre in the ISU tests. That was lower than what was observed in 2023. In 2023, the range was 2.5 to 43.6 pounds per acre. Lower levels in the fall of 2024 were likely due to a wetter growing season that led to higher corn yields (increased nitrogen uptake) and higher environmental loss.
Less carryover N for 2025
Soil nitrogen at zero-to-12-inch soil depth also positively correlates with soil nitrogen at 12 to 24 inches. Low soil N at zero to 12 inches can be indicative of low soil N at greater depths, Archontoulis says.
Furthermore, the majority of total inorganic nitrogen in last fall’s tests was nitrate-N rather than ammonium-N. The nitrate form of N is more susceptible to leaching and loss from the soil. The ammonium form of N is attracted to soil particles, making it more stable and less subject to loss.
For 2025, farmers can use this information, Archontoulis says. Keep in mind that in 96% of the fields sampled last fall, soil nitrogen content was less than 20 pounds of N per acre. You can use this for guidance when deciding your nitrogen fertilizer rate per acre to apply for the 2025 corn crop.
How much N to apply for 2025
When you use the N-FACT online decision support system to figure your optimum nitrogen application rate to apply this spring, it asks you several questions. One is to estimate the amount of post-harvest soil nitrate that was in the field last fall.
The first step using N-FACT is to select your location by clicking on a map of Iowa counties. Then, it asks you to choose the previous crop that was grown in the field. Are you growing corn after corn or corn after soybeans? You also select the expected precipitation for the 2025 growing season. You can choose average, above-average or below-average rainfall for your area.
Next, it asks how much post-harvest soil nitrate was in the field last fall. You can choose 20 pounds or less per acre, 20 to 70 pounds, or more than 70 pounds per acre. Select your estimated planting date for 2025 and your nitrogen fertilizer and corn prices. Finally, click on the “Estimate my optimal N rate” tab.
Residual soil nitrate varies
The term “residual soil nitrate" refers to the available N that’s carried over from one crop year to the next.
“While our soil-sampling results in our on-farm Iowa Nitrogen Initiative tests for soil nitrate is indicative of statewide patterns, keep in mind that every field and soil type is different,” Archontoulis says. “We encourage farmers and agronomists to sample the soil, consider the details of your farming operations, and use the N-FACT online digital tool when deciding on nitrogen rates to apply for 2025.”
N-FACT is a new online decision support program developed by the INI and ISU. It’s available for free to help Iowa farmers find the nitrogen application rate that’s right for their fields and conditions.
“Applying the correct rate of nitrogen varies by soil type, anticipated crop-year weather, residual soil nitrogen, crop rotation, planting date and fertilizer cost/crop pricing,” Archontoulis says. “Using N-FACT helps maximize productivity, profitability and environmental performance.”